The present invention relates to a thermal shut-off device used suitably for a secondary battery pack and a battery pack using the thermal shut-off device.
FIG. 9 shows a battery pack proposed by Japanese Patent Provisional Publication No. 7-153499. This battery pack incorporates a safety device a having a configuration in which a bimetallic switch b, a resistor c connected in parallel to the bimetallic switch b, and a resistor d connected in series to the bimetallic switch b are incorporated in a casing, not shown.
In this battery pack, if an excessive charging current flows in through a charging terminal e, Joule heat is generated in the resistor d, so that the bimetallic switch b is turned off to stop the charging to an incorporated battery f. If the bimetallic switch b is turned off, the off state of the bimetallic switch b is maintained by the heat generated in the resistors c and d.
FIGS. 10 and 11 show safety shut-off devices proposed by Japanese Patent Provisional Publication No. 63-503020. The shut-off device shown in FIG. 10 has a configuration in which a resistor i is connected in parallel to a bimetallic switch h connected in series to a load g, and a resistor k is interposed between a common connection point between the load g and the bimetallic switch h and the switch j.
In this device, if the switch j is closed, Joule heat is generated in the resistor k to turn off the bimetallic switch h, so that the current carried to the load g is interrupted. If the bimetallic switch h is turned off, the off state of the bimetallic switch h is maintained by the heat generated in the resistors i and k.
The shut-off device shown in FIG. 11 has a configuration in which resistors m and n connected in series to one another are connected in parallel to the bimetallic switch h, and the switch j is connected to a common connection point of the resistors m and n.
In this device, if the switch j is closed, Joule heat is generated in the resistors m and n to turn off the bimetallic switch h, with the result that the current carried to the load g is interrupted. If the bimetallic switch h is turned off, the off state of the bimetallic switch h is maintained by the heat generated in the resistor m.
The safety device shown in FIG. 9 has following problems.
(1) In order to maintain the off state of the bimetallic switch b by means of the heat generated in the resistor c, the impedance of the battery f, which is a load, must be low. This means that when the battery f is short-circuited externally, the on state is maintained. In the recent condition in which a battery pack is used, a secondary battery pack is scarcely removed from equipment such as a computer, so that the external short circuit of the battery occurs in fewer cases. Therefore, the function as a safety device is greatly limited.
(2) In recent years, a battery that requires exact charging/discharging control, such as a lithium battery, has been used frequently for a second battery pack. In the case of such a battery, overvoltage and overheating due to overcharging create a danger, so that multiple safety measures are taken against these phenomena.
As one example, both a safety device using a bimetallic switch and a temperature fuse are used for a protective control circuit. However, since the function of the safety device is greatly limited as described in the above item (1), the temperature fuse finally performs the shutting-off operation.
However, the temperature fuse cannot be reused once it performs the shutting-off operation. Therefore, even if the temperature fuse is operated by misuse, the expensive battery pack becomes incapable of being used.
(3) Since the resistor c is connected in parallel to the bimetallic switch b, complete electrical shutoff is not effected even if the bimetallic switch b is turned off. Therefore, after the bimetallic switch b is turned off, a phenomenon takes place in which a current flows into the battery f from a charger, not shown, connected to the charging terminal e or inversely a current leaks from the battery f to the equipment. This phenomenon is undesirable in terms of safety.
On the other hand, the shut-off device shown in FIG. 10 is configured so that the resistor i is connected in parallel to the bimetallic switch h, and the shut-off device shown in FIG. 11 is configured so that a series composite resistor consisting of the resistors m and n is connected in parallel to the bimetallic switch h.
Therefore, in these shut-off devices, complete electrical shutoff is not effected even if the bimetallic switch h is turned off, so that a current flows to the load g even after the bimetallic switch h has been turned off. That is to say, these shut-off devices pose the same problem as those described in the above item (3).
The present invention has been made in view of the above situation. Accordingly, an object of the present invention is to provide a thermal shut-off device capable of shutting-off operation under various abnormal conditions such as overcurrent, overvoltage, etc., maintenance of a shut-off state, reuse after an abnormality is eliminated, and complete electrical shutoff.
Another object of the present invention is to provide a battery pack using the above-described thermal shut-off device.
To achieve the above object, the present invention provides a thermal shut-off device comprising contacts interposed between first and second terminals, the contacts being opened/closed by the displacement of a heat responsive element, and a heat generating resistor interposed between the first terminal and a third terminal, wherein the heat responsive element is displaced by heat generated when a current is caused to flow in the heat generating resistor via the third terminal.
In the thermal shut-off device, a PTC element can be used as the heat generating resistor.
Also, the heat generating resistor can be formed into a film form, and the film-form resistor can be disposed so as to be in contact with the heat responsive element.
This thermal shut-off device achieves the following effects.
(1) Since a shutting-off operation can be performed based on various pieces of information and the shut-off state can be maintained by using one heat generating resistor, the device can be made small in size and low in cost, and the application range thereof can be broadened.
(2) Since no resistor is connected in parallel to the contacts to be opened/closed, a reliable electrical shut-off property can be provided. Therefore, the reliability as a protector is improved.
Also, the present invention provides a battery pack incorporating a thermal shut-off device comprising contacts interposed between first and second terminals, the contacts being opened/closed by the displacement of a heat responsive element, and a heat generating resistor interposed between the first terminal and a third terminal; and a chargeable battery, wherein the first terminal of the thermal shut-off device is connected to a first terminal for external connection, the second terminal thereof is connected to a second terminal for external connection via the battery, and the third terminal thereof is connected to a third terminal for external connection; and a current is caused to flow in the heat generating resistor via the third terminal for external connection, whereby the contacts are opened by the displacement of the heat responsive element caused by the heat generated by the heat generating resistor.
This battery pack achieves the following effects.
(3) An opening/closing operation of the thermal shut-off device can be performed and the open state can be maintained by an operation from a charger or external equipment such as a computer. Therefore, not only a protective operation against short circuit etc. but also a variety of protective operations based on various pieces of information can be performed.
(4) Since electrical shutoff can be effected reliably, the inflow and outflow of a current into and out of the battery during the time when the shutoff is continued is prevented. As a result, safety is improved.
(5) The battery pack has an economical advantage that it can be reused by disconnecting a heat generating power source.
Further, the present invention provides a battery pack incorporating a thermal shut-off device comprising contacts interposed between first and second terminals, the contacts being opened/closed by the displacement of a heat responsive element, and a heat generating resistor interposed between the first terminal and a third terminal; a chargeable battery; and abnormality detecting means for detecting the condition of the battery to close a switch element if the condition is abnormal, wherein the second terminal of the thermal shut-off device is connected to a first terminal for external connection, the first terminal thereof is connected to a second terminal for external connection via the battery, and the third terminal thereof is connected to the switch element; and a current from the battery is caused to flow to the heat generating resistor by closing the switch element, whereby the contacts are opened by the displacement of the heat responsive element caused by the heat generated by the heat generating resistor.
In this battery pack, abnormality detecting means can detect the condition of the battery based on the current of the battery.
Also, the abnormality detecting means can detect the condition of the battery based on the voltage of the battery.
Further, the abnormality detecting means can detect the condition of the battery based on the temperature of the battery.
This battery pack achieves the following effects in addition to the effect described in the above item (4).
(6) Since the shut-off state of the thermal shut-off device is maintained by the heat generation of the resistor caused by the discharge current of the battery, the thermal shut-off device automatically returns to the closed state when the discharge current of the battery decreases down to a predetermined value. Therefore, a returning operation of the thermal shut-off device is needed.
(7) A variety of protective operations can be performed, and also the battery pack can be reused.